The mixture of necessary nutrients can be used as a liquid medium, or a solidifying agent can be added. "Agar agar" is a natural polysaccharide produced by marine algae and is the most commonly used solidifying agent added to media (end concentration usually 1.5 % w/v). If hydrolysis of the agar is suspected, a silica gel is used as a replacement solidifying agent.
Before inoculation with the desired microorganisms, microbiological media and all materials coming into contact with it must be sterile. During any subsequent handling of the bacterial cultures, unwanted or contaminant organisms must be excluded employing aseptic techniques.
Sterilisation implies the complete destruction of all microorganisms including spores, this is accomplished by the use of heat, chemicals, radiation, filtration.
Denatures and coagulates vital proteins. There are various forms of heat sterilisation.
Inoculating wires or loops are sterilised by holding them in a Bunsen flame until they are red hot.
Bacteria are more readily destroyed by moist heat (steam) than dry heat. Usually used for the sterilisation of culture media, aqueous solutions and the destruction of discarded cultures. Air must first be removed in order to achieve the 121 °C necessary for successful sterilisation. This is accomplished by the use of an autoclave (the technical version of a pressure cooker), which follows automatic cycles of heating under pressure for the required time.
Usually employed for materials which could either be corroded by steam or must remain dry before use. These include metal instruments, glass petri dishes, flasks and pipettes and cotton wool. In practice, dry heat sterilisation requires longer time intervals and higher temperatures than steam sterilisation, e.g.steam sterilisation 121°C for 15mins or dry heat sterilisation 160°C for 120 minutes.
Usually employed for delicate equipment such as optical instruments and electrical devices which would be damaged by heat. Due to the toxicity of the chemicals used, this is not the most popular form of sterilisation. Chemicals employed include: gaseous ethylene oxide, which alkylates amino, sulfhydryl, carboxyl and hydroxyl groups of microbial cell compounds; formaldehyde, used as a fumigant; and hydrogen peroxide vapour used in aseptic packaging.
Employed for heat-sensitive materials and for environmental samples such as soil and sediment where structural changes caused by heat need to be avoided. Two forms of radiation are used:
Initiates the excitation of atoms which in nucleic acids leads to fatal mutations. UV light cannot penetrate materials so is used mainly for surface treatments e.g. Iaminar flow benches, and air and water.
Can penetrate samples, causing ionization within cells. Gamma radiation generated through a 60Co a-source is used to sterilise complex matrices such as soil and foodstuff. Microorganisms show increased resistance to radiation under anoxic conditions (2-5x) and also in frozen samples.
Filtration sterilisation operates through the exclusion rather than destruction of microorganisms. It is safe for the user and is employed for sensitive liquids and gases. Three types of filters are currently in use:
These are made of columns packed with fibrous materials such as glass wool or cotton wool. The twisting and turning fibres entrap particles and so act as filters; they show little resistance to flow and are used mainly for gases or as pre-filters for membrane filters which are easily clogged.
Act by screening out particles. Their effectiveness depends on the size of the membrane pores and the electrostatic attractions present. The most commonly used filters in microbiology are usually made of cellulose acetate or cellulose nitrate.
Size of filter pores required to screen out:
Yeast 0.45 -1.2 µm
Bacteria 0.2 µm
Viruses and mycoplasmas 0.01-0.1µm
Membrane filtration is usually employed for heat-sensitive substances, e.g. vitamin solutions; the filters are heat-sterilised before use.
Nucleation Track (Nuclepore) Filters
These filters consist of very thin polycarbonate films which have been treated with nuclear radiation and then etched with a chemical to create very uniform vertical holes. They are employed for the same material as membrane filters but have the disadvantage that they are more easily clogged.
Table of the most commonly used Media Supplements, Methods of Sterilisation, Solubilities